\documentclass{standalone}
\usepackage{tikz}
\usepackage{ctex,siunitx}
\setCJKmainfont{Noto Serif CJK SC}
\usepackage{tkz-euclide}
\usepackage{amsmath}
\usepackage{wasysym}
\usepackage{circuitikz}
\usetikzlibrary{patterns, calc}
\usetikzlibrary {decorations.pathmorphing, decorations.pathreplacing, decorations.shapes}
\ctikzset{bipoles/length=1cm}
\begin{document}
\small
\begin{circuitikz}[>=latex, scale=0.8,european]
  % \useasboundingbox(0.9,0)rectangle(5.1,5);
  \draw (0,6)--(2,6) to [R=,name=R1] (2,4) to [pR,name=hb] (2,2) to [R,name=R3] (2,0)--(0,0);
  \node at (R1.center)[left=1mm]{$R_1$};
  \node at (hb.center)[left=1mm]{$R_2$};
  \node at (R3.center)[left=1mm]{$R_3$};
  \draw (2,0)--(4,0)node[right]{$b$};
  \draw (hb.wiper)--(4,3)node[right]{$a$};
  \draw [fill=white](4,3) circle (2pt);
  \draw [fill=white](4,0) circle (2pt);      
  \draw [fill=white](0,0) circle (2pt);
  \draw [fill=white](0,6) circle (2pt);   
  \draw[<->](0,.1) to node [left]{$U$} (0,6-.1);
  \node at (2.5,2.7){$P$};
\end{circuitikz}
\end{document}